1. Field of the Invention
The present invention relates to a thermally bonded fibrous wet laid web containing a specific bicomponent fiber. This thermally bonded fibrous wet laid web not only has increased web strength, but also is found to provide greater web uniformity. Furthermore, the web is found to be much softer than a regular paper web. In particular, the bicomponent fiber consists essentially of a first component consisting of polyester or polyamide and a second component consisting of linear low density polyethylene and grafted high density polyethylene grafted with maleic acid or maleic anhydride. The thermally bonded fibrous wet laid web may further include a matrix fiber selected from a group consisting of cellulose paper making fibers, cellulose acetate fibers, glass fibers, polyester fibers, ceramic fibers, metal fibers, mineral wool fibers, polyamide fibers, and other naturally occurring fibers.
2. Prior Art
In the prior art processes of making wet laid webs or paper from fibers of whatever source, it is customary to suspend previously beaten fibers, or what is generally known as pulp, in an aqueous medium for delivery to a sheet-forming device, such as a Fourdrinier wire. This fiber containing aqueous dispersion is commonly referred to in the art as a furnish. One troublesome problem at this stage of making wet laid fibrous webs, is the tendency for the fibers to clump, coagulate or settle in the aqueous vehicle. This condition is generally referred to as flocculation, and greatly impedes the attainment of uniform web formation. That is, flocculation causes a nonuniform distribution of fibers in the paper product produced therefrom and manifests not only a mottled, uneven appearance, but is also defective in such important physical properties as tear, burst, and tensile strength. Another problem in making wet laid fibrous webs is a tendency of the fibers to float to the surface of the furnish.
For the manufacture of fibrous wet laid webs from conventionally used fibers such as cellulose, methods are known for attaining uniform dispersion of the fibers and reducing an even preventing the occurrence of flocculation. One of the more effective means has been to add a small amount of karaya gum to the fiber furnish. However, this has proved unsuccessful in various applications but other agents such as carboxymethyl cellulose or polyacrylamide have been used to attain the desired result of the cellulose in the furnish.
Fibrous wet laid webs may also be made from other natural or synthetic fibers in addition to the wood cellulose paper-making fibers. A water furnish of the fibers is generally made up with an associative thickener and a dispersant. The cellulose pulp is dispersed in water prior to adding the dispersant, followed by the addition of the associative thickener in an amount in the range up to 150 pounds per ton of dry fiber making up the water furnish and then the addition and dispersion of the natural and/or synthetic fibers. Finally, the dispersion of mixed fibers in a water carrier is diluted to the desired headbox consistency and dispensed onto the forming wire of a conventional paper-making machine. An anti-foam agent may be added to the dispersion to prevent foaming, if necessary, and a wetting agent may be employed to assist in wetting the fibers if desired. A bonded fibrous web may be formed from the fiber furnish on a high speed conventional Fourdrinier paper making machine to produce a strong, thermally bonded fibrous wet laid web.
In prior art processes for wet lay wherein the textile staple fibers are polyester fibers, water-based binders are generally added to the process to insure adhesion between the cellulose fibers and the polyester fibers. Generally, from about 4% to about 35% binder material is employed. On of the problems encountered using a water based binder is the binder leaches out of the resultant web in such applications as filters. Addition of binders increases cost and results in environmental problems. Furthermore, latex binders have a short shelf life and require special storage conditions. Also, the latex binders may be sensitive to the condition of the diluent water employed.
It is well known to blend bicomponent fibers with natural and synthetic fibers in dry processes of making nonwoven fabrics. For example, in European Patent Application No. 0 070 164 to Fekete et al there is disclosed a low density, high absorbent thermobonded, nonwoven fabric comprising a staple length polyester/polyethylene bicomponent fiber and short length natural cellulose fibers. The U.S. Pat. No. 4,160,159 to Samejima discloses an absorbent fabric containing wood pulp combined with short-length, heat fusible fibers. Although these patents disclose the use of the combination of bicomponent fibers and cellulose fibers, the disclosure is not directed to a wet lay application. Many problems arise in attempting to incorporate a heat fusible fiber such as a bicomponent fiber into a wet lay fibrous web.
Such nonwoven textile fabrics are normally manufactured by laying down one or more fibrous layers or webs of textile length fibers by dry textile carding techniques which normally align the majority of the individual fibers more or less generally in the machine direction. The individual textile length fibers of these carded fibrous webs are then bonded by conventional bonding (heating) techniques, such as, for example by point pattern bonding, whereby a unitary, self-sustaining nonwoven textile fabric is obtained.
Such manufacturing techniques, however, are relatively slow and it has been desired that manufacturing processes having greater production rates be devised. Additionally, it is to be noted that such dry textile carding and bonding techniques are normally applicable only to fibers having a textile cardable length of at least about 1/2 inch and preferably longer and are not applicable to short fibers such as wood pulp fibers which have very short lengths of from about 1/6 inch down to about 1/25 inch or less.
More recently, the manufacture of nonwoven textile fabrics has been done by wet forming technique on conventional or modified paper making or similar machines. Such manufacturing techniques advantageously have much higher production rates and are also applicable to very short fibers such as wood pulp fiber. Unfortunately, difficulties are often encountered in the use of textile length fibers in such wet forming manufacturing techniques.
Problems encountered in attempting to incorporate a heat fusible fiber such as a bicomponent fiber into a wet lay process is attaining uniform dispersion of the bicomponent fiber as well as attaining a thermally bonded web with sufficient strength such that the thermally bonded web is usable. It has been found in the past that bicomponent fibers containing a sheath of high density polyethylene (HDPE) and a core of polyester are difficult to uniformly disperse in wet lay solutions. When dispersion of fibers has been attained, fibrous webs produced therefrom have been found to have lacked the desired strength.
European Patent Application 0 311 860 discloses a bicomponent fiber having a polyester or polyamide core and a sheath component consisting of a copolymer straight-chain low density polyethylene; and the bicomponent fiber can be formed into a web through the use of known methods of making nonwoven fabrics including wet laying. The copolymer polyethylene is defined as consisting of ethylene and at least one member selected from the class consisting of an unsaturated carboxylic acid, a derivative from said carboxylic acid and a carboxylic acid and a carboxylic acid anhydride. The application fails to provide any details regarding the copolymer polyethylene into a wet lay process or the resulting properties of the web produced therefrom.
There remains a need to develop a thermally bonded wet lay fibrous web including a suitable heat fusible bicomponent filament which will not only increase the strength of the web, but also avoid problems associated with adding binders.